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Side effects

Note: less information about side effects is available for GC376, Molnupiravir and Remdesivir as they are currently used less often and have less formal study data available.

Cutaneous Sores, Lumps, Abscesses

This is the most commonly seen side effect with GS likely due to the low pH.  Sores are also seen to a lesser extent with GC376.

  • Sores

    • These can look quite ugly but generally heal well with little​ intervention.   In some cases, the sores have been quite large but it is extremely rare for cats to have require surgery for resolution. We recommend generally leaving sores alone other than other than removing hair from around the site, disinfecting it and preventing the cat from scratching it.  Some owners have had positive results with Duoxo calm or chlorhexidine mousse or Vetericyn spray.  Many cats on FIP treatment will wear shirts or recovery suits to prevent scratching or otherwise disturbing sores.

  • Lumps

      • Usually this is adipose tissue and does not require any intervention.  

  • Abscesses

    • These are often sterile but occasionally are infected and require a course of antibiotics​


Allergic reactions


Allergic reactions to FIP antiviral medications are very rare but occasionally reported.  In most cases the allergy appears to be to something in the drug preparation other than the antiviral itself, therefore switching to a different formulation or administration method (ie. injection to oral, or vice-versa) often resolves the issue.

The most common allergy manifestation is a rash, pruritus, acne, or other skin conditions.  In some reported cases it has been fairly severe.

A few cases of anaphylaxis have been reported associated with GS injections.

Elevated kidney values

Mild and transient BUN/creatinine elevation is reported relatively frequently with treatment using GS-441524.  In a few cats, primarily older cats or cats with pre-existing kidney disease more severe elevations can be seen.  


Anecdotally it is not uncommon for cats to have persistently elevated SDMA and some loss of urine concentrating ability for months post-treatment. There have been no studies on the long-term effects of GS on cat kidneys and assessment of risk is further complicated by the fact that the use of this drug is new and most cats are only a few years out from treatment. Given that without treatment cats will die of FIP, possible long-term consequences from GS administration seem much less concerning than the more immediate threat of FIP but cannot be dismissed entirely.  Post treatment monitoring of kidney values and urinalysis may be warranted in these cats. 


Subcutaneous fluid administration several times a week can help manage this during treatment, and often kidney values return to baseline once treatment has concluded.

Dental Development Delays

Treatment with GC376 has been noted to delay eruption of permanent teeth in cats treated before 16–18 weeks of age.

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